1. Field of the Invention
This invention relates to an information recording medium such as an optical disc, a magnetooptical disc or the like, and an information reproducing apparatus which is suitably used to reproduce information from the information recording medium.
2. Description of Related Art
For a conventional optical disc used in a CAV (constant angular velocity) mode, a servo byte section is periodically provided at a prescribed position on each track, and clock pits to generate reference clocks and wobbled pits for tracking are formed in the servo byte section. The reference clocks (channel clocks) are formed in accordance with the clock pits, and information is digitally recorded in accordance with pits whose length is set to be integer times of the period of the reference clocks.
A system which is used in a CLV (Constant Linear Velocity) such as a CD (Compact Disc) is formed with no clock pit, however, the length of recorded pits and the interval of the pits are set to be integer times (any one of nine length levels from about 0.9 .mu.m to 3.3 .mu.m for CD) of the period (0.3 .mu.m) of the reference clocks (channel clocks) (so-called self-clock system is adopted), and clock components contained in reproduced RF signals are extracted to cut out the recorded information on a pit basis.
For a video disc which is the same type of optical disc as the CD, video signals are FM-modulated, recorded and reproduced on the basis of the difference in length between pits which are designed in a finer size than those for CDs. This will be described in more detail representatively using a case where signals are recorded in the CAV mode on a 55 mm-radius disc.
In the case of the video disc, the lightest portion of recording information is recorded with signals of 9.3 MHz while the darkest portion is recorded with signals of 7.6 MHz, and these signals correspond to the lengths of 1.075 .mu.m and 1.316 .mu.m on the 55 mm-radius disc, respectively. It is well known that a very beautiful picture can be reproduced by reproducing information from the disc which is recorded in the above recording mode. Assuming that the picture thus obtained is represented in 128 gradation levels (that is, variation in brightness of the picture is represented in 128 levels), this means that pits constituting the picture thus obtained are recorded while varying the pit period in 128 or more levels, and reproduced. That is, the minute variation in pit length and pit interval as represented by the following equation reflects the video signals: EQU (1.316 .mu.m-1.075 .mu.m)/128=0.002 .mu.m
Although such minute variation in pit length is recordable as described above, the minimum variation unit in pit length for CDs must be set to 0.3 .mu.m which is a larger value than 0.002 .mu.m, and this is mainly caused by the fact that recording and reproducing methods which have been recently used for the CDs are not optimum.
In Japanese Patent Application No. 3-167585, the applicant of this application previously proposed a recording and reproducing method in which digital information is recorded by shifting stepwise the front edge or rear edge position of an information pit from a predetermined reference position in accordance with recording information. According to the recording and reproducing method, the variation in pit length and pit edge position can be detected with very high precision, so that the digital information can be recorded with the very minute variation which have hitherto seemed to be impossible, and thus a higher density recording can be performed.
FIG. 1 shows a timing chart showing the principle of stepwise shifting the edge position of each pit to record information, which was previously proposed by the applicant.
As shown in FIG. 1, a recording signal shown at a middle stage of FIG. 1, is generated while subjected to a pulse width modulation (PWM) in accordance with recording data, and on the basis of the recording signal thus generated are formed pits each of which has the length corresponding to the distance between zero-cross points thereof. Through this operation, the edge position of each pit can be stepwise shifted from a reference position as indicated by a reference clock. In accordance with this shift, any one of data of eight stages (positional levels) from "0" to "7" (represented by three bits) can be recorded for one edge.
FIG. 2 shows a timing chart showing the principle of reproducing information from the pits thus recorded. An RF signal shown at a top stage of FIG. 2 which is reproduced from an information recording medium is greatly amplified to obtain a binary RF signal. Clock pits are formed on a disc having information recorded thereon, and thus a reference clock shown at a third stage of FIG. 2 is generated on the basis of the clock pits. Further, a sawtooth-wave signal shown at a bottom stage of FIG. 2 is generated in synchronism with the reference clock. The edge position of each information pit is detected by detecting a timing at which the sawtooth-wave signal and the binary RF signal cross over each other.
In addition to the above proposed method, the applicant also previously proposed a method of two-dimensionally decoding data recorded in the above manner. In this method, educational pits are beforehand formed on an optical disc, and 64 (=8.times.8) combinations from (0,0) to (7,7) can be provided for a combination (M,N) of the front-end edge M and the rear-end edge N of each educational pit. The educational pits thus recorded are reproduced, and then reference points are mapped on a RAM in correspondence with the reproduced levels of the educational pits as shown in FIG. 3.
Thereafter, ordinary data pits are reproduced to sample the levels of the reproduced RF signal at two positions of the front-end edge and the rear-end edge of each data pit and obtain a point on the RAM which is specified by the two levels. Thereafter, a reference point which is nearest to the point is obtained, and the data are decoded on the assumption that the data pit corresponding to the above point has the same edge combination as the educational pit corresponding to the nearest reference point.
In such a method that the reference points corresponding to the educational pits are mapped on a memory and then the nearest reference point is obtained to decode the data, however, when an intersymbol interference state varies, the content of the RAM, that is, all the positions of the reference points must be re-written in accordance with the variation of the intersymbol interference state. For example, when an optical disc has any skew, the intersymbol interference is varied at high speed in accordance with rotation of the optical disc, however, it is impossible to re-write data on the RAM at high speed in correspondence with the high-speed rotation of the optical disc.
Further, when the level at a sampling point for a data pit is represented with 8 bits, two-points information can be mapped on a RAM having an address space of 16 bits. The number of sampling points is preferably increased as many as possible. However, if the number of the sampling points is set to three or more, the scale of the RAM is unpracticably large. For example for three sampling points, an address space of 24 bits is required, and in this case 16 Mbits are required for capacity of the RAM. For four sampling points, the scale of the RAM is required to have capacity of 256 times of the above case. It is practically impossible to use a RAM having such a large scale capacity.
Still further, the method as described above needs the educational pits, and the educational pits are preferably formed in a larger amount. In addition, in order to learn the intersymbol interference varying at high speed, the educational pits are required to be frequently recorded. However, when a large number of educational pits are recorded as described above, a recording area for data pits which are to be originally recorded and reproduced is reduced, and thus the capacity of the disc is reduced.